1. Field of the Invention
The present invention relates to an infrared sensor and a manufacturing method thereof, and particularly to an infrared sensor that detects an infrared ray by using a thermosensitive resistor whose resistance value changes when an infrared ray enters it, and a manufacturing method thereof.
2. Description of the Related Art
In general, thermal type infrared sensors use a thermosensitive resistor such as a bolometer thin film, and make it absorb infrared rays radiated from an object and transform the infrared into heat to cause the temperature of the thermosensitive resistor to rise and its resistance value to change, thereby detecting the amount of the received infrared rays based on the change of the resistance value.
Infrared sensor arrays are also known, which are built up of infrared sensors integrated two-dimensionally in the form of a matrix, on whose pixels are arranged the respective infrared sensors. Infrared sensor arrays can enable photography during nighttime by letting the infrared sensors on the respective pixels detect the amount of infrared that enters the respective pixels.
However, since thermal type infrared sensors detect the amount of infrared based on a change of the temperature of the thermosensitive resistor, they may not be able to correctly detect the amount of infrared if there occurs a change of the ambient temperature (the temperature of the environment surrounding the infrared sensors) that might cause a drift that influences the output of the infrared sensors.
It is possible to overcome this problem by, for example, equipping the infrared sensor array with a temperature regulator. With temperature regulation on each infrared sensor by the temperature regulator, any drift of the output from the infrared sensors due to a change of the ambient temperature can be suppressed. However, this method requires furnishing of a temperature regulator and entails a product price increase. Hence, Unexamined Japanese Patent Publication KOKAI Publication No. H10-227689 describes an infrared sensor that suppresses any drift of its output without using a temperature regulator.
This infrared sensor includes a first thermosensitive element whose resistance value changes in response to an entering infrared ray, and a second thermosensitive element whose resistance barely changes in response to an entering infrared ray. The first thermosensitive element detects the amount of the entering infrared ray. The voltage output from the first thermosensitive element is corrected by the voltage output from the second thermosensitive element in a manner to cancel any influence from a change of the ambient temperature.
The first thermosensitive element has a heat-insulated structure that prohibits heat transfer from a bolometer thin film to a silicon substrate. Accordingly, the temperature of the first thermosensitive element rises above the ambient temperature when an infrared ray enters there. As a result, the first thermosensitive element comes to have a resistance value that is determined by the ambient temperature and the amount of the entering infrared ray.
On the other hand, the second thermosensitive element has a structure that allows heat transfer from the bolometer thin film to the silicon substrate. Since the heat produced in the bolometer thin film in response to an entering infrared ray escapes into the silicon substrate, the temperature of the second thermosensitive element does not almost change when an infrared ray enters. As a result, the second thermosensitive element comes to have a resistance value that is determined by the ambient temperature.
This infrared sensor lets electric currents of the same level flow through the first and second thermosensitive elements, and reads the difference between the resulting voltages in them. The infrared sensor detects the amount of the entering infrared ray by offsetting the influence of the ambient temperature change based on the voltage difference.
The first thermosensitive element is formed of a bolometer thin film that is disposed on a hollow support base to provide a clearance between the bolometer thin film and the silicon substrate. This clearance hinders the bolometer thin film from transferring its heat to the silicon substrate, thermally insulating the bolometer thin film from the silicon substrate. For this reason, the temperature of the first thermosensitive element changes in response to an entering infrared ray, and the resistance value of the bolometer thin film of the first thermosensitive element changes based on both of the incidence of the infrared ray and a change of the ambient temperature.
Meanwhile, the second thermosensitive element is formed of a bolometer thin film formed on a support base as well as the first thermosensitive element, but the support base is not hollow to allow thermal communication between the bolometer thin film and the silicon substrate. Therefore, the bolometer thin film easily transfers its heat to the silicon substrate. The bolometer thin film of the first thermosensitive element and that of the second thermosensitive element have the same shape and almost equal Temperature Coefficient of Resistance (TCR) values.
When an infrared ray enters the bolometer thin film of the second thermosensitive element, the heat produced by the infrared ray transfers to the heat sink silicon substrate, leaving the resistance value of the bolometer thin film of the second thermosensitive element almost unchanged. That is, the resistance value of the bolometer thin film of the second thermosensitive element changes based only on a change of the ambient temperature.
The support base for the bolometer thin film of the second thermosensitive element may be formed thin or may be formed to a height of almost the same level with the top of the clearance of the first thermosensitive element.
However, if the support base for the bolometer thin film of the second thermosensitive element is formed thinly, the members that constitute the first thermosensitive element and the members that constitute the second thermosensitive element may come at different heights, and this might cause a reduced projection exposure machine, which is used for forming a resist mask on the members, to miss the focus and allow a poor accuracy in the resist mask pattern. Hence, when manufacturing an infrared sensor that includes first and second thermosensitive elements, it is advantageous in terms of processing convenience that the support base of the bolometer thin film of the second thermosensitive element be formed to have a height almost the same as that of the clearance of the first thermosensitive element.
In the case of forming the support base of the bolometer thin film of the second thermosensitive element to have a height almost the same as that of the clearance of the first thermosensitive element, a sacrifice layer is formed on the silicon substrate, and the bolometer thin film of the first and second thermosensitive elements, and a protection member are formed on the sacrifice layer. Then, a slit is formed in the first thermosensitive element to expose the sacrifice layer such that only the sacrifice layer that underlies the first thermosensitive element is etched out and a clearance is formed. The bolometer thin film of the second thermosensitive element is thermally connected to the silicon substrate via the sacrifice layer.
The sacrifice layer needs to be of a material that can be easily removed with no unfavorable influence on the other structural members. Hence, the sacrifice layer is formed of the resin such as polyimide, which can be removed by dry etching is used. However, the resin such as polyimide have a small heat transfer rate, and cannot therefore quickly let out to the silicon substrate the heat produced in the bolometer thin film by an entering infrared ray. The resin such as polyimide have a problem that they cannot make the temperature of the second thermosensitive element quickly follow the temperature of the silicon substrate.
To improve the heat transfer, the sacrifice layer may be made of silicon, polysilicon, metal, etc. However, these materials generally need to be removed by wet etching. Wet etching entails difficulty in draining the etching solution that gets stocked in the clearance. Further, wet etching has a problem that the clearance might be deformed to bring the bolometer thin film of the first thermosensitive element in contact with the silicon substrate, while the etching solution is drained.